2.5. Swelling Ratio Measurements

The swelling ratio was calculated by measuring the ratio between the swollen (V_s) and unswollen (V_r) hydrogel volumes. A buoyancy kit was attached to an analytical scale to determine volume changes. A total of 30 μL of hydrogels was prepared into disks, as previously described, and the volume was measured immediately after gelation. This was followed by volume measurements by weighing the hydrogels in air and after they had been swollen in serum-free αMEM for 1, 2, 4, 48, and 72 h. Five independent replicates for each gel type were prepared from individual polymer aliquots (n = 5). All measurements were prepared at room temperature.

Based on the swelling ratio, the mesh size of an ideal swollen polymer network can be estimated as²⁷

where υ_2,s = 1/Q_v and is the mean unperturbed end-to-end distance of the polymer given by

where l is the average bond length (taken as 1.54 Å for vinyl polymers²⁸), C_n is the characteristic ratio (4.0 for PEG²⁹), $\bar{M}$_c the molecular weight between two cross-links (assumed to be 11,500 g/mol), and M_r is the molecular weight per repeat unit (44 g/mol). It should be noted that eqs 1 and 2 give a general estimate of the mesh size, assuming an even distribution of cross-linking density, and do not account for network inhomogeneities, such as dangling ends, primary loops, or entrapped entanglements.